For an optical semiconductor module, especially for an optical semiconductor laser module used in an optical fiber amplifier, a package is used in order to accommodate hermetically a driver IC, an optical semiconductor such as a laser diode (LD), and so.
Generally, a package 1, as shown in FIG. 5, includes a frame 2 that is made by an Fe—Ni—Co alloy (trade name; covar) and so on and jointed to a bottom plate 3 that is made of an Fe—Ni—Co alloy, an Fe—Ni alloy (trade name; 42 alloy), or a metal matrix composite of CuW and so on. In particular, CuW is used as a material of the bottom plate 3 in the package 1 that needs heat radiation due to large power consumption.
A frame 2, which is a part of a side wall of the package 1, has a ceramic feedthrough 4. Lead frame (not shown) made of covar are formed on the ceramic feedthrough 4. Another type of package has a structure in which the frame 2 is made of a ceramic insulator and integrated with the ceramic feedthrough 4. A light transmission window (not shown) to allow light to penetrate therethrough between the inside and the outside of the package is provided in the frame 2 of the package 1.
Parts, such as the frame 2, bottom plate 3, and ceramic feedthrough 4 are assembled and jointed by silver brazing or soldering. The assembled package 1 is gold plated for ease of soldering in the step of assembling into a semiconductor module as well as for making hermetic sealing with a cap and for protection against package corrosion. A square ring 5 made of covar that is necessary for welding or soldering of the cap is provided on the upper end face of the frame 2 of the package 1.
In the process of mounting an optical semiconductor device to the package 1, an electronic cooling element 6 such as a Peltier element is mounted on the bottom plate 3, and a circuit board on which the optical semiconductor device and the like are mounted beforehand is jointed to the electronic cooling element. As shown in FIG. 4, the electronic cooling element has a structure in which n-type thermoelectric elements 6a (for example, BiTeSe) and p-type thermoelectric elements 6b (for example, BiTeSb) are arranged alternately in a matrix, and couples of neighboring thermoelectric elements 6a and 6b are connected either at their top faces or at their bottom faces through a metal pad 8 provided on a ceramic substrate 7a or 7b. 
An output electrode 8a of the electronic cooling element 6 is connected to a Cu lead 9, and the optical semiconductor device and feedthrough of the package are connected through Au wires. Then, after a cap (not shown) is mounted on a square ring 5 and sealed, an optical fiber and a light transmission window of the package 1 are aligned and welded together by a laser such as a YAG laser. Thus, an optical semiconductor module was completed.
In the optical semiconductor module, in order to prevent decline of optical output in the optical semiconductor device such as a laser diode (LD) or to maintain the isothermal state in the light guide device, the temperature of the optical semiconductor device is controlled by means of electronic cooling element such as the Peltier element. The endothermic amount of the electronic cooling element is nearly proportionate to the junction area between the electronic cooling element and the bottom plate of the package.
Generally, the frame 2 of the package 1 has inside and outside juts of the ceramic feedthrough as shown in FIG. 5. Therefore, for mounting an electronic cooling element 6 on the bottom plate 3, the electronic cooling element 6 is passed vertically through the space between the inside juts 4a of the ceramics feedthrough 4 of both sides of frame 2, and their jointing is carried out by soldering in a hydrogen atmosphere.
Therefore, only the electronic cooling element which can pass between the inside juts 4a of the ceramic feedthrough 4 can be mounted, resulting in limitation in the area of a ceramic circuit board 7b of the electronic cooling element 6 to be jointed to the bottom plate 3. Thus, the area of junction between the electronic cooling element 6 and the bottom plate of the conventional package 1 is only about 70% of the area of the bottom plate. Moreover, even if an electronic cooling element 6 is inserted in a slant manner between the inside juts 4a of the ceramic feedthrough 4, it is impossible to mount the electronic cooling element 6 that can have a junction area equivalent to more than 75% of the bottom plate area.